The disclosure of Japanese Patent Application No. 11-329891 filed on Nov. 19, 1999, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to vehicular engine starting control apparatus and vehicular engine starting control methods for a vehicle having an engine and a motor generator as power sources.
2. Description of Related Art
Hybrid vehicles having an engine and a motor generator as power sources for driving the vehicle are known. For example, as disclosed in Japanese Laid-Open Patent Application No 9-170533 (which corresponds to U.S. Pat. No. 5,934,395), some vehicles of this type have an engine started by means of a motor generator and others have an engine started by a separate starter (a dedicated motor for starting the engine).
A hybrid vehicle performs an iterative operation of automatically stopping and restarting the engine (hereinafter referred to as xe2x80x9cintermittent operationxe2x80x9d for convenience of explanation). Thus, the engine is either started through operation by the vehicle driver (started by switching an ignition key from OFF to ON) or automatically restarted during intermittent operation. However, in the aforementioned vehicles, a suitable selection in terms of the characteristics of the starter and the motor generator has not always been made.
It is one object of the invention to provide a vehicular engine starting control apparatus for a vehicle equipped with both a starter and a motor generator wherein startability of the engine can be improved by starting the engine while suitably selecting between the starter and the motor generator.
In order to solve the aforementioned and/or other problems, according to a first aspect of the invention there is provided a vehicular engine starting control apparatus for a vehicle having an engine which outputs power to a drive shaft of the vehicle, a motor generator which inputs power to and outputs power from a power transmission path that extends from an output shaft the engine to the drive shaft, and a starter which is coupled to the output shaft of the engine. The vehicular starting control apparatus includes a controller that: (1) receives an initial engine starting command if an ignition key has been switched from OFF to ON; (2) receives an engine restarting command upon fulfillment of a predetermined restarting condition during automatic stop of the engine; (3) issues a command to start the engine by using the starter upon issuance of the initial engine starting command; and (4) issues a command to start the engine by using the motor generator upon issuance of the engine restarting command.
In general, the starter and the motor generator are intrinsically different in function. Although they are both electric motors, they are different in type and performance.
The starter (ST) can reliably start the engine under any circumstance (e.g., even in a frigid state). The starter is usually constructed of a DC motor and driven at a low speed and a large torque with a low voltage and large electric power. Thus, since the starter is capable of generating a large torque, the engine can be started reliably even at a low temperature. On the other hand, the starter has disadvantages such as large power consumption, inadequate responsive characteristics and strong oscillation.
The motor generator (MG) is used as an electric motor to assist or replace the engine when driving the vehicle, and is used as a generator to regenerate coasting energy during deceleration (which is used to charge a battery that supplies energy to the motor generator when it is used as an electric motor). Thus, because of mechanical characteristics of the motor generator, the torque that can be generated thereby is smaller in comparison with the starter. The motor generator is driven at a high speed and a low torque with a high voltage and small electric power. Thus, even when the engine is repeatedly stopped and automatically restarted irrespective of the driver""s intention as in the case of intermittent operation, the motor generator makes it possible to start the engine without causing a sense of incongruity to the driver. However, the motor generator cannot generate a large torque and is thus unsuited for the starting operation at a low temperature, for example. Especially in a frigid state where the battery itself cannot function properly, the motor generator is disadvantageous in that it causes further deterioration of startability.
In the first aspect of the invention, the engine starting operation that is performed when the driver switches the ignition ON and the engine restarting operation that is automatically performed during intermittent operation are distinguished from each other. If the ignition key has been switched from OFF to ON, the engine is started by the starter. On the other hand, the engine is restarted by the motor generator while the vehicle is being driven (i.e., restarted during intermittent operation).
In many cases, the engine is initially started by switching the ignition key ON when the temperature is low. Because this operation is performed by the starter, the engine starting operation can be performed with high reliability. Because this starting operation is based on the driver""s intention, oscillation or noise can be tolerated to some extent. Such oscillation or noise sometimes helps the driver confirm that the engine definitely has been started. Further, since the engine is restarted by means of the motor generator during intermittent operation, the engine restarting operation can be performed tranquilly and smoothly (without causing a sense of incongruity to the driver). Therefore, it is possible to sufficiently exploit advantages of using the starter and the motor generator while counterbalancing their disadvantages.
According to a second aspect of the invention, there is provided a vehicular engine starting control apparatus for a vehicle having an engine which outputs power to a drive shaft of the vehicle, a motor generator which inputs power to and outputs power from a power transmission path that extends from an output shaft of the engine to the drive shaft, and a starter which is coupled to the output shaft of the engine. The vehicular engine starting control apparatus includes a temperature detector which detects a temperature of the engine, and a controller which: (1) issues a command to start the engine with the starter if the detected temperature of the engine is equal to or lower than a predetermined temperature; and (2) issues a command to start the engine with the motor generator if the detected temperature of the engine is higher than the predetermined temperature.
In the second aspect of the invention, it is determined whether the engine is to be started with the starter or with the motor generator depending on a detected temperature of the engine. That is, since the engine cannot be started smoothly when the temperature of the engine is low, the engine is started with the starter. Thus, while sacrificing tranquility to some extent, startability of the engine is ensured with high reliability. On the other hand, since the engine can be started smoothly when the temperature of the engine is high, the engine is started with the motor generator. Thus, startability of the engine is ensured with tranquility, smoothness and good responsive characteristics.
In the case of the second aspect of the invention, when starting the engine, the starter or the motor generator is selected depending on a temperature of the engine, without regard to whether the engine is started through operation of the ignition key or the engine is restarted during intermittent operation. This aspect of the invention is especially useful when the engine is started during intermittent operation. That is, when the engine is started during intermittent operation, use of the motor generator usually causes no problem. For example, if the vehicle has been stationary for a long time in a cold period with the engine being stopped, the engine becomes low in temperature. Thus, it may be more or less difficult to start the engine with the motor generator. Intermittent operation (automatic stop and restarting of the engine) is performed under various circumstances irrespective of the driver""s intention. Therefore, such a situation in which the engine cannot be started smoothly during intermittent operation is undesirable.
In this respect, according to the second aspect of the invention, if the temperature of the engine lowers, the starting operation with the motor generator is automatically switched to the starting operation with the starter. Hence, good startability of the engine can be ensured irrespective of a temperature of the engine.
For example, if the predetermined temperature is set equal to or lower than a ordinary temperature, the engine can be started by the motor generator in place of the starter when the vehicle is used at the ordinary temperature. Therefore, it is possible to start the engine more tranquilly in comparison with the starting operation with the starter.
Although the temperature of the engine can be measured through installation of a dedicated temperature sensor, a temperature of an engine coolant, namely, a value detected by a coolant temperature sensor can be used instead. It is also possible to substitute an outside air temperature or a temperature of intake air for the temperature of the engine. Further, a temperature of an on-vehicle battery can also be utilized.
According to a third aspect of the invention, there is provided a vehicular engine starting control apparatus for a vehicle having an engine which outputs power to a drive shaft of the vehicle, a motor generator which inputs power to and outputs power from a power transmission path that extends from an output shaft of the engine to the drive shaft, and a starter which is coupled to the output shaft of the engine. The vehicular engine starting control apparatus includes a temperature detector which detects a temperature of the engine, and a controller that: (1) receives an initial starting command if an ignition key has been switched from OFF to ON; (2) issues a command to start the engine with the starter if the detected temperature of the engine is lower than a predetermined temperature upon issuance of the initial engine starting command; and (3) issues to start the engine with the motor generator if the detected temperature of the engine is higher than the predetermined temperature upon issuance of the initial engine starting command.
That is, also in this case, when starting the engine the starter or the motor generator is selected depending on a temperature of the engine. However, this starting operation is performed only if the ignition switch has been switched ON.
If the ignition key has been switched ON, it can be regarded that the starting operation has been performed based on the driver""s intention. Thus, there is no need to give careful consideration to noise or oscillation. However, under a circumstance where the engine can be started with the motor generator, the motor generator is selected to start the engine, for it becomes possible to ensure more tranquility and better responsive characteristics. Thus, for example, the engine can be started at an ordinary temperature tranquilly and smoothly.
When starting the engine with the motor generator, the controller in the second and third aspects of the invention may be designed to set a starting speed to a lower speed in accordance with a fall in temperature of the engine.
In general, when the engine is started with the starter, the starting speed of the engine is usually equal to approximately 200 to 300 rpm. On the other hand, when the engine is started with the motor generator, the starting speed of the engine is set to approximately 800 to 1000 rpm. In an engine speed range above 800 rpm, oscillation of the vehicle tends to be strong. Thus, by instantaneously raising the engine speed to a speed exceeding the oscillation range, oscillation of the vehicle can be prevented. Consequently, it becomes possible to start the engine tranquilly.
As described above, in the case where the engine starting operation is selectively performed either by the starter or by the motor generator depending on the temperature of the engine, there is a great difference between a starting speed obtained by the starter and a starting speed obtained by the motor generator. Thus, if the temperature exceeds a certain temperature (a predetermined temperature), the starting speed for starting the engine jumps abruptly. This may cause a sense of incongruity to the driver.
For the purpose of minimizing such a sense of incongruity, the control is performed such that the starting speed obtained by the motor generator approaches the speed obtained by the starter (on the low-speed side). This not only serves to eliminate a sense of incongruity but also reduces the burden on the motor generator resulting from a rise in engine speed by reducing the starting speed obtained by the motor generator when the temperature is low and causing the engine to rotate autonomously at an early stage. That is, when the temperature is low, it is burdensome to raise the engine speed to 800 to 1000 rpm with the motor generator, because of a deterioration of performance of the battery and an increase in engine friction. Hence, if the engine speed has risen to a relatively low speed, the motor generator ends its role as a starter. That is, while neglecting oscillation to some extent, an attempt is made to reduce power consumption.
In this case, the relation between engine coolant temperature and starting speed obtained by the motor generator may be changed linearly. However, with a view to avoiding to the utmost an engine oscillation band that exists around 800 rpm, the characteristic curve for determining the relation therebetween may be realized as a convex curve or a concave curve. It is also possible to adopt a characteristic curve with a fall so that the oscillation range can be skipped (as will later be described).
Aspects of the invention are not limited to a control apparatus for a hybrid vehicle as described above. For example, other aspects of the invention include a hybrid vehicle or a hybrid system, and a method of controlling a hybrid vehicle or a hybrid system.